The lites of glass in an insulating unit must be held apart at the
appropriate distance by spacers. In addition to keeping the glass lites
separated, the spacer system must serve a number of functions:

provide a moisture barrier that prevents passage of water or water vapor that would fog the unit;

provide a gas-tight seal that prevents the loss of any special low-conductance gas in the air space;

create an insulating barrier that reduces the formation of interior condensation at the edge.

The standard solution for insulating glass units (IGUs) is the use of metal spacers and sealants. These spacers, typically aluminum, also contain a desiccant that absorbs residual moisture. The spacer is sealed to the two glass layers with organic sealants that provide structural support and act as a moisture barrier. There are two generic systems for such IGUs: a single-seal spacer and a double-seal system (see figure to the right).

In the single-seal system, an organic sealant, typically a butyl material, is applied behind the spacer and serves to hold the unit together and prevent moisture intrusion. These seals are normally not adequate to contain special low-conductance gases.

In a double-seal system, a primary sealant, typically butyl, seals the spacer to the glass to prevent moisture migration and gas loss, and a secondary backing sealant, often silicone, provides structural strength. When sputtered low-E coatings are used with double-seal systems, the coating must be removed from the edge first (edge deletion) to provide a better edge seal.

Since aluminum is an excellent heat conductor, the aluminum spacer used in most standard edge systems represents a significant thermal "short circuit" at the IGU edge, reducing the benefits of improved glazings. Window manufacturers have developed a series of innovative edge systems to address this problem, including solutions that depend on material substitutions as well as radical new designs. One approach to reducing heat loss replaces the aluminum spacer with a less conductive metal, e.g., stainless steel, and changing the cross-sectional shape of the spacer.

Another approach is to replace the metal with a design that uses better insulating materials. An example is an insulating silicone foam spacer that incorporates a desiccant and has a high-strength adhesive at its edges to bond to glass. The foam is backed with a secondary sealant. Both extruded vinyl and pultruded fiberglass spacers have also been used in place of metal designs.

Warm edge spacers have become increasingly important as manufacturers switch from conventional double glazing to higher-performance glazing. For purposes of determining the overall window U-factor, the edge spacer has a thermal effect that extends beyond the physical size of the spacer to a band about 2-½ inches wide. The contribution of this 2-½-inch-wide "edge of glass" to the total window U-factor depends on the size of the window. For instance, edge of glass effects are more important for smaller windows, which have a proportionately larger glass edge area.

A more significant benefit may be the rise in interior surface temperature at the bottom edge of the window, which is most subject to condensation. With an outside temperature of 0 degrees Fahrenheit, a thermally improved spacer could result in temperature increases of 6–8° Fahrenheit at the window sightline or 4–6° Fahrenheit at a point one inch in from the sightline, which is an important improvement. As new highly insulating multiple layer windows are developed, the improved edge spacer becomes an even more important element.

The window on the left is a double glazing with low-E and an insulating spacer. The window on right also uses low-E and a partially insulating spacer. The difference is that on the right the window uses three different low-E coatings in a quadruple layer design and the air inside the panes has been replaced with more insulating krypton gas. Such a high performance window is called a "superwindow". These windows are being cooled on the back side with wind at -17.8°C (0°F). Image courtesy Lawrence Berkeley National Laboratory.